What kind of base did you put under the concrete, nicely compacted road base material? If the underneath is not providing a good suppport I think you might be marginal, but the fiber will certainly help.
I have two smaller machines, about 5000lbs each on a 2" - 3" slab that has nothing but topsoil under it and what I did was use pads of steel plate to spread the load over a larger area than the little pucks the levelling screws sit on. Between the plate and the concrete I put a 1/2" thick piece of dense polyurethane foam; my theory for this was that it compresses and spreads the load evenly over the concrete surface, not concentrating at just the high spots where the steel touches. So far a few years on I have not seen any extra cracks in the floor.